1. Field of the Invention
The present invention relates to a method for forming a high duality cuprous oxide film on a desired substrate for a semiconductor device by way of a electro deposition process and a process for the production of a semiconductor device such as a photovoltaic element including a solar cell.
2. Related Background Art
Cuprous oxide films had been used as semiconductors from old times. Particularly, they had been used in the production of rectifiers in the 1920s, and in the 1930s, they had been used in the production of photodetectors. Since then, instead of such cuprous oxide film, films of selenium, cadmium sulfide, germanium, or silicon have been used, where the cuprous oxide film has been seldom used. However, it should be noted that a cuprous oxide film can be relatively easily formed at a reasonable production cost and it is not toxic, and therefore, it is still a promising material for use in the production of a photovoltaic element such as a solar cell.
By the way, the formation of a cuprous oxide film has been conducted mainly by way of thermal oxidation or anodic oxidation of a copper metal material. These methods have advantages such that their film-forming process is simple and a cuprous oxide film is formed having a high purity in a chemical sense. However, particularly for the thermal oxidation method, there are disadvantages in that the substrate temperature during the film formation is at least 300.degree. C., usually more than 500.degree. C. and because of this, only a substrate which is highly heat-resistant can be used; and a cuprous oxide film formed on a given substrate is liable to peel off from the substrate due to a difference between the coefficient of thermal expansion of the cuprous oxide film and that of the substrate. Besides, cuprous oxide films formed by any of these two methods are problematic in that because their formation is based on conversion of crystal structure from copper metal into cuprous oxide, they are likely to contain minute defects or to be distorted. In this respect, these cuprous oxide films are not satisfactory in terms of the electric characteristics.
Now, U.S. Pat. No. 2,081,121 discloses a method of depositing a cuprous oxide film utilizing cathodic reduction of an alkali solution of copper sulfate or the like. According to this method, because particularly high temperature is not required and no crystal structure's conversion is involved in the film formation, it seems that a good cuprous oxide film having few defects and which is slight in distortion can be formed. However, it is difficult for this method to form a desirable cuprous oxide film having a p-type property.
Besides, it is considered that reactive sputtering ban be employed in the deposition of a cuprous oxide film. However, in this case, the film composition is greatly varied due to a change in the partial pressure of oxygen, and because of this, it is difficult to repeatedly form an aimed cuprous oxide film. (see, N. A. Economou et als., Proc. Photov. Sol. Energy Conf. (1997), p. 1180)
In D. Trivich et als., 13th IEEE Photov. Spec. Conf., p. 174, IEEE, New York, there is described that a solar cell in which a cuprous oxide film is used theoretically could attain a photoelectric conversion efficiency of 13%. However, as of the present time, there has not been realized a practically usable solar cell in which a cuprous oxide film is used and which provides practically acceptable solar cell characteristics.
In accordance with any of the conventional techniques for the formation of a cuprous oxide film, it is difficult to repeatedly form a large area cuprous oxide film having a good property. Particularly, it is almost impossible to produce a large area semiconductor device (element) having excellent characteristics at a reasonable production cost by efficiently forming a large area and high quality cuprous oxide film on a large area substrate of low heat-resistance.